Electrochemical method for manufacturing methyl ethyl ketone

ABSTRACT

It is provided a process for the preparation of methyl ethyl ketone by electroreduction of acetoin in aqueous media using a high hydrogen overvoltage cathode made of lead, the process comprising the steps of a) forming a solution by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium, and b) electrolyzing said solution continuously or discontinuously in an electrochemical reactor by applying a voltage between an anode and the cathode using a direct current power supply at a current density from 500 to 5000 A/m2.

This application claims the benefit of European Patent Application EP16382424.6 filed on Sep. 14, 2016.

TECHNICAL FIELD

The present invention is related to an electrochemical method for manufacturing methyl ethyl ketone (also known as 2-butanone and MEK) by electroreduction of acetoin (also known as 3-hydroxybutanone) in a solution formed by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium, using a high hydrogen overvoltage cathode in both divided and undivided cells.

BACKGROUND ART

MEK is an important chemical widely used industrially as a solvent in the vinyl resins and synthetic rubber industries. Currently, MEK is industrially produced by dehydrogenation of 2-butanol over cooper and zinc oxide catalysts at 400-500° C. and pressures lower than 0.4 MPa, such as disclosed in U.S. Pat. No. 4,075,128. Other chemical methods described in the prior art are the Wacker liquid phase oxidation of butenes at about 85° C. and 0.69 MPa as disclosed in U.S. Pat. No. 5,506,363; and the dehydration of 2,3-butanediol using acidic catalysts as reported in Zhao et al. “Catalytic dehydration of 2,3-butanediol over P/HZSM-5: effect of catalyst, reaction temperature and reactant configuration on rearrangement products”, RSC Adv., 2016, Vol. 14, pp. 16988-16995.

These methods involve a serious environmental burden and use non-renewable fossil resources as raw materials, with the exception of that starting from 2,3-butanediol which can be obtained by fermentation of sugars. Nevertheless, this last method operates at a high temperature so that it is intensive in energy consumption. Therefore, there is a need for non-polluting new methods for manufacturing MEK starting from renewable raw materials and able to work at low temperatures and pressures.

U.S. Pat. No. 3,247,085 discloses an electrochemical process for making MEK by electro-oxidation of 1-butene.

Baizer et al. “Electrochemical conversion of 2,3-butanediol to 2-butanone in undivided flow cells: a paired synthesis”, J. Appl. Electrochem, 1987, Vol. 14, pp. 197-208 discloses a procedure for converting 2,3-butanediol in ca. 10% aqueous solution to MEK by passing it through a porous anode at which it is selectively oxidized to acetoin by electrogenerated NaBrO and then pumping to a porous cathode at which it is reduced to MEK. The acetoin formed in solution by oxidation of 2,3-butanediol with the electrogenerated NaBrO is electroreduced to MEK in the cathode. However, this process has no industrial applicability due to its very low current density of 20 A/m², far below the useful industrial ones ranging from at least 500 A/m² to about 5000 A/m². Therefore, the process of Baizer et al. results in very poor productivities requiring a huge capital investment. Another strong current drawback of this process from an environmental standpoint is that Hg-based cathodes are used as well as the presence of NaBrO within the electrolyzed solution. Additionally, Baizer et al. states that an increase in the current density above 20 A/m² caused more H₂ evolution and resulted in poor current efficiency as well as high cell voltage due to the gas trapped inside the cell, concluding that the paired reaction should be run at a low current density (10 or 20 A/m²) to obtain relatively high current efficiencies.

WO2016097122 discloses a process for manufacturing 2,3-butanediol by electroreduction of 3-hydroxybutanone in an aqueous media by using porous Pt or Ni cathodes. In the example comparative 1, MEK is obtained by electroreduction of 3-hydroxybutanone using a Sigracet® GDL-24BC cathode in a 64.0% selectivity for a 75.7% conversion of 3-hydroxybutanone. However, productivity, i.e. the kg of MEK produced per hour and per m² of electrode (cathode) area (kg-MEK/h/m²), a key parameter directly related to the industrial productivity (the higher P_(MEK) the lower the capital investment), is low for practical use.

Thus, there continues to be a need of an industrially scalable process which allows obtaining MEK with an increased productivity.

SUMMARY OF THE INVENTION

Inventors have found a new process for the preparation of MEK that overcomes and/or minimizes some of the drawbacks of the processes disclosed in the prior art. Particularly, an economical and industrially scalable process for the production of MEK with higher productivities than the ones obtained by the processes of the prior art is provided. As it can be seen from the examples, by means of the new process, MEK is obtained in an aqueous solution by electroreduction of acetoin at room temperature and ambient pressure, at the current densities required for industrial feasibility, and with a significantly high productivity.

Thus, the invention relates to a process for the preparation of methyl ethyl ketone (MEK) by electroreduction of acetoin in aqueous media using a high hydrogen overvoltage cathode made of lead, the process comprising the steps of:

-   -   a) forming a solution by mixing acetoin with an aqueous medium         and a supporting electrolyte soluble in such a medium, and     -   b) electrolyzing said solution continuously or discontinuously         in an electrochemical reactor by applying a voltage between an         anode and the cathode using a direct current power supply at a         current density from 500 to 5000 A/m², particularly of 2500,         2000, 1500, or 1000 A/m².

DETAILED DESCRIPTION OF THE INVENTION

As used herein “hydrogenation catalyst” means a catalyst which is capable of catalysing the reduction by hydrogen of a group susceptible of being reduced in a bulk catholyte, wherein hydrogen was previously electrogenerated in the cathode by electroreduction of water. Thus, in the presence of a hydrogenation catalyst electrolysis is used for generating hydrogen not for electroreducing directly the group susceptible of being reduced. Examples of hydrogenation catalysts are supported noble metals (such as supported Pt, Pd, Ru Ir and Rh), Raney Ni, and supported Ni.

Acetoin has an asymmetric carbon and consequently it is a chiral molecule. Any one of the stereoisomers as well as their mixtures can be used as a raw material in the process of the present invention. Accordingly, throughout the present invention the term acetoin encompasses its enantiomers as well as mixtures thereof in any proportions, e.g. a racemic mixture or an enantiomerically enriched mixture of its enantiomers.

Acetoin can be obtained by fermentation of an aqueous solution of glucose, sacarose or molasses as disclosed in ES2352633, wherein the microorganism carrying out the bioconversion is a mutant strain of Lactococcus lactis lactis. By means of such a process the manufacturing cost of acetoin is low enough for making the electrosynthesis of MEK from acetoin economically feasible.

As used herein, the terms “cell”, “electrochemical cell” and “electrochemical reactor” are interchangeable.

As used herein, “aqueous medium” means 100 wt % water, or a mixture of water with a fully or partially water-miscible solvent in which the amount of water is from 50 to 99 wt %, particularly from 70 and 99 wt %, and more particularly from 85 and 99 wt %. Suitable fully or partially water-miscible solvents are those which are not electroactive under the electrolysis conditions of the present invention. Examples of said solvents, but not limited to, are alcohols such as methanol, ethanol, propanol and isopropanol; ethers such as tetrahydrofuran and dioxane; and nitriles such as acetonitrile.

As mentioned above the present invention relates to a process for the preparation of MEK by electroreduction of acetoin in aqueous media using a high hydrogen overvoltage cathode made of lead. Particularly, the reaction is carried out in the absence of a hydrogenation catalyst.

In a particular embodiment, the cathode material is lead as a flat sheet or deposited in a porous support such as a carbon felt, carbon foam, or a similar material.

The electrochemical reactor used in the process of the present invention can be any one of those known by a person skilled in the art such as a tank-type electrochemical reactor or a flow-through filter press-type electrochemical reactor. In a particular embodiment of the process of the invention, the electrochemical reactor is a flow-through filter press-type electrochemical reactor. The electrochemical reactor can be divided or undivided, with this last configuration being the most preferred because leads to both a lower power consumption and a lower capital investment. If a divided electrochemical reactor is used, anode and cathode are separated by a material preventing mixing of the anolyte (the acetoin-free solution being fed through the anodic compartment, e.g. an aqueous solution of sulfuric acid) and the catholyte (the acetoin-containing solution being fed through the cathode compartment) while allows the flow of ions transporting electricity in solution. A cation exchange membrane is the most preferred separating material for divided electrochemical reactors. Examples of cation exchange membranes include, but are not limited to, any one of those marketed under the trademark of Nafion® such as, e.g., Nafion® N-324 and Nafion® N-424.

In a particular embodiment of the process of the invention, as anodic materials (anode) carbon steel, and platinum supported on titanium (Pt/Ti) and iridium-based DSA® (dimensionally stable anodes) are used in the method of the present invention. They can be used in non-porous flat form and as perforated materials such as nets, metal meshes, lamellae, shaped webs and grids.

The electroreduction of acetoin to MEK according to the present invention is performed in the presence of a supporting electrolyte added to adjust the conductivity of the electrolysis solution and/or to control the selectivity of the reaction. In a particular embodiment of the process of the invention, the amount of the supporting electrolyte is generally adjusted to a level from 0.1 to 20 wt %, particularly from about 1 to about 15 wt %, and more particularly from about 5 to about 10 wt %, based on the total mass of the solution. Examples of supporting electrolytes in undivided cells and for catholyte when divided cells are used include, but are not limited to, ammonium and alkaline and alkaline earth metals salts of inorganic acids such as sulfuric, phosphoric and nitric acids, and ammonium quaternary salts, such as, e.g., tetraethyl ammonium bromide, chloride and sulfate, and tetrabutyl ammonium bromide, chloride and sulfate.

If the process of the present invention is carried out in a divided cell, additional supporting electrolytes for catholyte are ammonium and alkaline and alkaline-earth metals salts of hydrochloric acid, hydrobromic acid and hydrofluoric acid; and supporting electrolytes for anolyte include, but are not limited to, inorganic acids, such as sulfuric and phosphoric acids, as well as ammonium and alkaline and alkaline earth metals salts of said inorganic acids. Accordingly, in a particular embodiment, the process of the invention is carried out in a divided cell and the supporting electrolyte forming a solution with acetoin is selected from the group consisting of ammonium and alkaline and alkaline earth metal salts of an inorganic acid, ammonium quaternary salts, and mixtures thereof, and the supporting electrolyte for anolyte is a non-oxidizable inorganic acid.

pH of electrolyte in undivided cells or pH of catholyte in divided cells can be from 2.5 to 7. Accordingly, in a particular embodiment of the process of the invention, the pH of the solution formed by mixing acetoin with the aqueous medium and the supporting electrolyte soluble in such a medium is from 2.5 and 7, particularly from 3 to 7, and more particularly from 4 to 7. pH adjustment can be done by adding a suitable acid such as phosphoric or sulfuric acid, or base such as sodium or potassium hydroxide. If pH is lower than 2.5 current efficiency decreases due hydrogen evolution by electroreduction of protons. If pH is higher than 7 the selectivity of the reaction is negatively affected due to aldol condensations of both acetoin and MEK.

Acetoin concentration in the solution, formed by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium, to be electrolyzed is at least 10 g/L, particularly at least 25 g/L, more particularly at least 50 g/L and the most particularly at least 100 g/L, based on the total volume of solution to be electrolyzed.

In a particular embodiment of the process of the invention, the amount of electricity circulated for electroreducing acetoin to MEK is from 50% to 125% of the theoretical one for obtaining a 100% conversion of acetoin assuming a current efficiency of 100% (2 faradays per mol of acetoin), more particularly from 55% and 100%, and most particularly from 60% and 75%.

In a particular embodiment of the process of the invention, the temperature for electroreduction of acetoin to MEK is from 10° C. to 70° C. Particularly the electrolysis temperature is room temperature.

In a particular embodiment, after electrolysis completion MEK is separated by vacuum evaporation and the aqueous phase is loaded with fresh acetoin for restoring its initial concentration and the electrolysis is resumed.

In a particular embodiment, MEK is continuously removed from the aqueous medium during electrolysis by vacuum evaporation. Thus, the MEK-containing aqueous medium exiting from the electrochemical reactor is heated to a temperature from 40° C. to 50° C. and sent to a vacuum evaporator where MEK is evaporated and condensed. The MEK-depleted aqueous medium is cooled down in a heat exchanger to the electrolysis temperature and sent back to the electrochemical reactor where the remaining acetoin is electroreduced to MEK. When the acetoin concentration decreases below a level from 40 to 50% of the initial one, the initial concentration is restored by adding fresh acetoin.

In another particular embodiment, MEK is continuously removed from the aqueous medium during electrolysis by liquid-liquid extraction using a water-insoluble inert solvent such as toluene, xylenes, tert-butyl methyl ether, and methyl isobutyl ketone. Other suitable solvents are easily recognizable by those skilled in the art.

In another particular embodiment, the process of the invention is carried out:

-   -   i) in one electrochemical reactor, or     -   ii) in at least two electrochemical reactors connected in series         in such a way that the solution, comprising a mixture of         unreacted acetoin, MEK, an aqueous medium, and a supporting         electrolyte soluble in such a medium, resulting from one         electrochemical reactor feeds the subsequent one.

If two or more electrochemical reactors connected in series are used both current density and circulated electrical charge decrease from the first electrochemical reactor to the last one. For instance, if two electrochemical reactors connected in series are used, the current density used in the first electrochemical reactor is higher than that used in the second electrochemical reactor; and the fraction of the circulated electrical charge in the first electrochemical reactor, relative to the total charge circulated through both electrochemical reactors, is higher than that in the second electrochemical reactor. In this way, electricity is more efficiently employed in electroreducing acetoin to MEK.

Throughout the description and claims the word “comprise” and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Furthermore, the word “comprise” encompasses the case of “consisting of”. The following examples are provided by way of illustration, and they are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of particular and preferred embodiments described herein.

EXAMPLES Example 1

A solution (60 mL) of acetoin (100 g/L) and KH₂PO₄ (5 wt %) in water was recirculated at a flow-rate of 2 L/min by means of a magnetic pump through the cathode compartment of a divided filter press cell consisting of a Ti-supported iridium oxide-based DSA flat sheet as an anode (20 cm²), a Nafion® N-324 cation exchange membrane for separating anode and cathode compartments, and a lead flat sheet as a cathode (20 cm²). Inter-electrode gap was 1.7 cm. An aqueous 5 wt % sulfuric acid solution was recirculated through the anode compartment by means of another magnetic pump. An electrical current was circulated (3 A, 1500 A/m²) by applying a voltage between anode and cathode using a DC Power Supply. Electrolysis was kept at room temperature (20-25° C.) for 73.01 min (100% of the theoretical charge for full conversion of acetoin assuming a current efficiency of 100%). Initial catholyte pH was 4.32 and final pH 3.75 (mean pH 4.04). After electrolysis completion, the catholyte solution (64 mL) contained an acetoin concentration of 27.2 g/L and a MEK concentration of 40.8 g/L, as shown by HPLC. Therefore, acetoin conversion was 71% (71% current efficiency) and MEK yield was 53.1% (53.1% current efficiency) resulting in a selectivity to MEK, the ratio between yield and conversion, of 74.9%. MEK productivity was 1.07 kg MEK/h/m².

Example 2

As in example 1, but using as catholyte a solution (60 mL) of acetoin (100 g/L), KH₂PO₄ (5 wt %) and tetraethyl ammonium bromide (1 wt %) in water. Initial catholyte pH was 4.31 and final one 4.23 (mean pH 4.27). After electrolysis completion, the catholyte solution (64 mL) contained an acetoin concentration of 25.1 g/L and a MEK concentration of 46.7 g/L, as shown by HPLC. Therefore, acetoin conversion was 73.2% (73.2% current efficiency) and MEK yield was 60.8% (60.8% current efficiency) resulting in a selectivity to MEK of 83.1%. MEK productivity was 1.23 kg MEK/h/m².

Example 3

As in example 1, but using as catholyte a solution (60 mL) of acetoin (100 g/L), KH₂PO₄ (5 wt %) and tetrabutyl ammonium bromide (0.5 wt %) in water. Initial catholyte pH was 4.32 and final one 6.68 (mean pH 5.50). After electrolysis completion, the catholyte solution (63 mL) contained an acetoin concentration of 11.7 g/L and a MEK concentration of 38.7 g/L, as shown by HPLC. Therefore, acetoin conversion was 87.8% (87.8% current efficiency) and MEK yield was 49.7% (49.7% current efficiency) resulting in a selectivity to MEK of 56.6%. MEK productivity was 1.00 kg MEK/h/m².

Example 4

As in example 1, but using as catholyte a solution (60 mL) of acetoin (100 g/L), KH₂PO₄ (5 wt %) in water adjusted to pH 7.0 with KOH. Final pH was 6.97 (mean pH 6.99). After electrolysis completion, the catholyte solution (63 mL) contained an acetoin concentration of 21.8 g/L and a MEK concentration of 42.4 g/L, as shown by HPLC. Therefore, acetoin conversion was 77.1% (77.1% current efficiency) and MEK yield was 54.3% (54.3% current efficiency) resulting in a selectivity to MEK of 70.4%. MEK productivity was 1.10 kg MEK/h/m².

Example 5

As in example 1, but using as catholyte a solution (60 mL) of acetoin (100 g/L), KH₂PO₄ (5 wt %) in water adjusted to pH 3.07 con H₂SO₄. Final pH was 2.64 (mean pH 2.86). After electrolysis completion, the catholyte solution (63 mL) contained an acetoin concentration of 20.6 g/L and a MEK concentration of 39.5 g/L, as shown by HPLC. Therefore, acetoin conversion was 78.4% (78.4% current efficiency) and MEK yield was 50.6% (50.6% current efficiency) resulting in a selectivity to MEK of 64.5%. MEK productivity was 1.02 kg MEK/h/m².

Example 6

As in example 1, but using as catholyte a solution (60 mL) of acetoin (100 g/L), KH₂PO₄ (5 wt %) in water adjusted to pH 5.5 with KOH, and a current density of 1000 A/m² (2 A, electrolysis time of 109.6 min corresponding to an electric charge of 100% relative to the theoretical one). Final pH was 5.53. After electrolysis completion, the catholyte solution (61 mL) contained an acetoin concentration of 11.6 g/L and a MEK concentration of 52.2 g/L, as shown by HPLC. Therefore, acetoin conversion was 88.2% (88.2% current efficiency) and MEK yield was 64.7% (64.7% current efficiency) resulting in a selectivity to MEK of 73.4%. MEK productivity was 0.87 kg MEK/h/m².

Example 7 (Comparative Example)

As in example 6, but using cadmium as a cathode. Final pH was 5.51. After electrolysis completion, the catholyte solution (63 mL) contained an acetoin concentration of 7.5 g/L and a MEK concentration of 45.6 g/L, as shown by HPLC. Therefore, acetoin conversion was 92.1% (92.1% current efficiency) and MEK yield was 58.4% (58.4% current efficiency) resulting in a selectivity to MEK of 63.5%. MEK productivity was 0.79 kg MEK/h/m².

Examples 8, 9 (Comparative), 10 (Comparative), and 11-24

These examples illustrate the influence of cathode material (examples 8, 9 (comparative), 10 (comparative), and 11-15), acetoin concentration (examples 8, 16 and 17; and 19 and 21), electric charge (examples 18-20) and temperature (examples 21-24). Experiments were performed as in example 1 (1500 A/m² and a divided cell) using as catholyte a solution (60 mL) of acetoin (in the concentration specified in Table 1) and KH₂PO₄ (in the concentration specified in Table 1) in water adjusted to pH 5.5 with KOH, by circulating an electric charge also specified in Table 1. pH was 5.5 and kept constant throughout the electrolysis. Results are given in Table 1, wherein the meaning of symbols is as follows:

-   -   E: Electrolyte (Catholyte for divided cells)     -   Q: electric charge, % of the theoretical charge for full         conversion of acetoin assuming a current efficiency of 100%,     -   C: Acetoin conversion,     -   S_(MEK). Selectivity to MEK,     -   η_(MEK): MEK current efficiency,     -   [Acetoin]_(i): initial acetoin concentration,     -   [MEK]_(f): final MEK concentration after electrolysis         completion,     -   Sigracet GDL-24BC/SS: A gas diffusion layer (SGL Group, The         Carbon Company) supported by gluing on 20 cm² of a stainless         steel sheet, Pb-X/GDL-24BC/SS: Pb electrodeposited on Sigracet         GDL-24BC/SS in an amount of X μg/cm² of geometric area.     -   P: MEK productivity     -   ΔP: Productivity increase (%) vs Comparative Example 1(%)

TABLE 1 Influence of cathode material, acetoin concentration, electric charge and temperature in divided cells P [Acetoin]_(i) Q T [MEK]_(f) C S_(MEK) η_(MEK) (kg MEK/ Ex. Cathode J (A/m²) (g/L) E (%) (° C.) g/L (%) (%) (%) h/m²) Δp (%) 8 Pb 1500 100 A 100 20 43.4 74.3 77.2 57.4 1.16 78.2 9 Cd 1500 100 A 100 20 39.6 91.8 55.4 50.8 1.02 57.7 10 Zn 1500 100 A 100 20 34.2 88.1 49.7 43.8 0.88 36.0 11 Pb-200/GDL-24BC/SS 1500 100 A 100 20 37.6 80.2 62.0 49.7 1.00 54.3 12 Pb-500/GDL-24BC/SS 1500 100 A 100 20 40.5 83.1 64.5 53.6 1.08 66.4 13 Pb-1000/GDL-24BC/SS 1500 100 A 100 20 41.4 81.9 66.8 54.7 1.10 69.8 14 Pb-5000/GDL-24BC/SS 1500 100 A 100 20 39 85.0 62.6 53.2 1.07 65.1 15 Pb-20000/GDL-24BC/SS 1500 100 A 100 20 44.4 98.3 59.8 58.7 1.18 82.2 16 Pb 1500 200 A 100 20 85.5 90.2 67.5 60.9 1.23 89.0 17 Pb 1500 300 A 100 20 116.1 86.3 64.8 55.9 1.13 73.5 18 Pb 1500 100 B 50 20 26.1 39.6 81.9 64.9 1.31 101.4 19 Pb 1500 100 B 75 20 37.7 67.4 56,9 51.1 1.03 58.6 20 Pb 1500 100 B 100 20 41.5 72.0 76.2 54.9 1.11 70.4 21 Pb 1500 200 B 75 12 85.8 75.2 78.9 79.1 1.60 145.5 22 Pb 1500 200 B 75 20 83.5 66.7 86.7 77.0 1.55 139.0 23 Pb 1500 200 B 75 35 75.1 73.2 64.7 63.1 1.27 95.9 24 Pb 1500 200 B 75 45 68.5 72.6 64.3 62.3 1.26 93.4 A: KH₂PO₄ (5 wt %) adjusted to pH 5.5 with KOH; B: KH₂PO₄ (10 wt %) adjusted to pH 5.5 with KOH.

Example 25 (Comparative, from WO2016097122)

A solution (60 mL) of 3-hydroxybutanone (101.1 g/L), KH₂PO₄ (2.5 wt %) and Na₂SO₄ (4 wt %) in water adjusted to pH 3.8 with phosphoric acid, was recirculated by means of a magnetic pump through an undivided filter press cell consisting of a Iridium oxide-based DSA anode (20 cm²) and a 20 cm² (geometric area) Sigracet® GDL-24BC cathode separated 0.8 cm each other by means of a PP separator. An electrical current was circulated (2 A, 1000 A/m²) by applying a voltage between anode and cathode using a DC Power Supply. Electrolysis was kept at room temperature (20-25° C.) for 1.90 h corresponding to 102.8% of the theoretical charge for full conversion of 3-hydroxybutanone assuming a current efficiency of 100%. Initial solution pH was 3.8 and final pH 3.7. After electrolysis completion, the electrolyzed solution (57.8 mL) contained a 3-hydroxybutanone concentration of 25.5 g/L and a methyl ethyl ketone concentration of 41.7 g/L, as shown by HPLC. Therefore, 3-hydroxybutanone conversion was 75.7% (73.6% current yield), MEK yield was 48.5% (a MEK selectivity of 64%) and MEK productivity was 0.65 kg MEK/h/m².

Example 26

As in Example 25 (comparative), but using a lead flat sheet instead of Sigracet® GDL-24BC as a cathode. 3-hydroxybutanone conversion was 82.3% (80.1% current yield), MEK yield was 62.1% (a MEK selectivity of 75.4%) and MEK productivity was 0.83 kg MEK/h/m², 27.7% higher than that obtained in Comparative Example 1.

Examples 27-31

Similarly as in Example 26, these examples show the good performance of the present process using an undivided cell. A solution (60 mL) of acetoin (200 g/L) and KH₂PO₄ (10 wt %) in water, adjusted to pH 5.5 with KOH, was recirculated at a flow-rate of 2 L/min by means of a magnetic pump through the compartment of an undivided filter press cell consisting of a Ti-supported iridium oxide-based DSA mesh as an anode (20 cm² geometric area) and a lead flat sheet as a cathode (20 cm²). Inter-electrode gap was 0.8 cm. An electrical current was circulated (at the current density, J (A/m²), given in Table 2) by applying a voltage between anode and cathode using a DC Power Supply. Electrical charge, Q, as % of the theoretical one was as given in Table 2, and temperature was 22° C. Results are given in Table 2.

TABLE 2 Results in an undivided cell. Meaning of symbols as in Table 1 P (kg- J Q [MEK]_(final) C S_(MEK) η_(MEK) MEK/ ΔP Ex. (A/m²) (%) (g/L) (%) (%) (%) h/m²) (%) 27 1500 75 85.2 72.5 71.8 69.4 1.40 115.4 28 1000 68 91.2 63.9 85.5 80.4 1.08 66.4 29 1000 75 98.2 74.1 79.5 78.6 1.06 62.6 30 1000 50 65.7 47.5 84.6 80.4 1.08 66.4 31 750 75 97.2 73.5 80.8 79.2 0.80 22.9

Example 32

As in example 28 (Table 2), but using a carbon steel (C: 0.40-0.50%; Mn: 0.50-0.80%; Si: 0.15-0.40%) anode instead a Ti-supported iridium oxide-based DSA mesh. Acetoin conversion was 68% (100% current efficiency) and MEK yield was 50.2% (50.2% current efficiency) resulting in a selectivity to MEK of 73.8%.

Example 33 (Comparative)

As example 9 (comparative; Table 1) except that the catholyte comprised 40 mL of acetoin (100 g/L), KH₂PO₄ (5 wt %) in water adjusted to pH 5.5 with KOH, and 20 mL of xylenes for extracting continuously MEK from the aqueous phase. After electrolysis completion, the concentration of acetoin in the catholyte aqueous phase (42 mL) was 7.95 g/L and the concentration of MEK was 18.75 g/L, while the acetoin concentration in the catholyte organic phase (15 mL) was 0 g/L and the MEK concentration was 67.7 g/L, as shown by HPLC. Therefore, acetoin conversion was 91.7% (91.7% current efficiency) and MEK yield was 55% (55% current efficiency) resulting in a selectivity to MEK of 60%. Thus, conversion was equal to that obtained in the absence of an extraction solvent, but selectivity to MEK was 8.3% higher. MEK productivity was 0.79 kg MEK/h/m², 8.8% higher than that in example 10. This comparative example shows the positive effect of removing continuously MEK by liquid-liquid extraction as electrolysis proceeds.

INDUSTRIAL APPLICABILITY

The above examples demonstrated the industrial applicability of the method of the present invention and its advantages. It can be operated at room temperature and ambient pressure under current densities (related to the process productivity, the higher the current density the higher the productivity, provided that the current efficiency keeps constant, or it decreases in a percentage lower than the increase in current density percentage) typically used in industrial electrochemical processes for manufacturing organics. Additionally, it works both in divided and undivided cells, with selectivities to MEK as high as 85.5% (see example 30, Table 2) in undivided cells or 86.7% (see example 26, Table 1) in divided cells, and with MEK productivities suitable for industrial production.

REFERENCES CITED IN THE APPLICATION

-   1. U.S. Pat. No. 4,075,128 -   2. U.S. Pat. No. 5,506,363 -   3. Zhao et al. “Catalytic dehydration of 2,3-butanediol over     P/HZSM-5: effect of catalyst, reaction temperature and reactant     configuration on rearrangement products”, RSC Adv., 2016, Vol. 14,     pp. 16988-16995. -   4. WO2016097122 -   5. U.S. Pat. No. 3,247,085 -   6. Baizer et al. “Electrochemical conversion of 2,3-butanediol to     2-butanone in undivided flow cells: a paired synthesis”, J. Appl.     Electrochem, 1987, Vol. 14, pp. 197-208 -   7. ES2352633 -   8. Popp FD and Schultz HP “Electrolytic reduction of organic     compounds” Electrolytic Reduction of Organic Compounds. Chem Rev,     1962, Vol. 62, pp: 19-40 

1. A process for the preparation of methyl ethyl ketone (MEK) by electroreduction of acetoin in aqueous media using a cathode made of lead, the process comprising the steps of: a) forming a solution by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium, and b) electrolyzing said solution continuously or discontinuously in an electrochemical reactor by applying a voltage between an anode and the cathode using a direct current power supply at a current density from 500 to 5000 A/m².
 2. The process according to claim 1, wherein the process is carried out in the absence of a hydrogenation catalyst which is a catalyst which is capable of catalysing the reduction by hydrogen of a group susceptible of being reduced in a bulk catholyte.
 3. (canceled)
 4. The process according to claim 1, wherein the cathode made of lead is lead as a flat sheet or deposited in a porous support such as a carbon felt, and carbon foam.
 5. The process according to claim 1, wherein the anode is selected from carbon steel, platinum supported on titanium, and iridium-based dimensionally stable anodes in non-porous flat form and as perforated materials such as nets, metal meshes, lamellae, shaped webs and grids.
 6. The process according to claim 1, wherein the aqueous medium comprises 100 wt % water or a mixture of water with a fully or partially water-miscible non-electroactive solvent in which the amount of water is from 50 to 99 wt %, particularly from 70 to 99 wt %, more particularly from 85 to 99 wt %.
 7. The process according to claim 1, wherein the electrochemical reactor is an undivided reactor.
 8. The process according to claim 1, wherein the supporting electrolyte forming a solution with acetoin is selected from the group comprising ammonium and alkaline and alkaline earth metal salts of inorganic acids, and ammonium quaternary salts, and mixtures thereof, and the supporting electrolytes for anolyte in divided cells are non-oxidizable inorganic acids.
 9. The process according to claim 8, wherein supporting electrolyte forming a solution with acetoin is in an amount from 0.1 to 20 wt %, particularly from 1 to 15 wt %, and more particularly from 5 to 10 wt %, based on the total mass of the solution.
 10. The process according to claim 1, wherein the solution formed by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium has a pH from 2.5 and 7, particularly from 3 to 7, and more particularly from 4 to
 7. 11. The process according to claim 1, wherein for electroreducing acetoin to MEK, an amount of electricity from 50% and 125% of the theoretical one for obtaining a 100% conversion of acetoin assuming a current efficiency of 100%, more particularly from 55% to 100%, and most particularly from 60% to 75%, is circulated.
 12. The process according to claim 1, wherein electroreduction is carried out at a temperature from 10° C. to 70° C., particularly room temperature.
 13. The process according to claim 1, wherein the MEK is continuously removed from the aqueous medium by vacuum evaporation.
 14. The process according to claim 1, wherein the MEK is continuously removed from the aqueous medium by liquid-liquid extraction using a water-insoluble inert solvent.
 15. The process according to claim 1, which is carried out: i) in one electrochemical reactor; or ii) in at least two electrochemical reactors connected in series in such a way that the solution, comprising a mixture of unreacted acetoin, MEK, an aqueous medium, and a supporting electrolyte soluble in such a medium, resulting from one electrochemical reactor feeds the subsequent one.
 16. The process according to claim 2, wherein the cathode material is lead as a flat sheet or deposited in a porous support such as a carbon felt, and carbon foam.
 17. The process according to claim 16, wherein the aqueous medium comprises 100 wt % water or a mixture of water with a fully or partially water-miscible non-electroactive solvent in which the amount of water is from 50 to 99 wt %, particularly from 70 to 99 wt %, more particularly from 85 to 99 wt %.
 18. The process according to claim 17, wherein the electrochemical reactor is an undivided reactor.
 19. The process according to claim 18, wherein the supporting electrolyte forming a solution with acetoin is selected from the group comprising ammonium and alkaline and alkaline earth metal salts of inorganic acids, and ammonium quaternary salts, and mixtures thereof, and the supporting electrolytes for anolyte in divided cells are non-oxidizable inorganic acids.
 20. The process according to claim 19, wherein the amount of the supporting electrolyte forming a solution with acetoin is from 0.1 to 20 wt %, particularly from 1 to 15 wt %, and more particularly from 5 to 10 wt %, based on the total mass of the solution.
 21. The process according to claim 20, wherein the solution formed by mixing acetoin with an aqueous medium and a supporting electrolyte soluble in such a medium has a pH from 2.5 and 7, particularly from 3 to 7, and more particularly from 4 to
 7. 